[PATCH] Improve internal diff error messages

I noticed some ugliness when I ran svn like this:

$ svn diff --internal-diff
svn: Error parsing diff options: Bad character specified on command line

Huh? I didn't specify a bad character on the command line. What
character, anyway?

The problem was that my configuation file specified GNU diff as the
default diff command and GNU diff options including "-U6" as the default
diff options.

The following patch improves the situation.

My only concern is that it assumes the option-parsing error messages
produced by APR will always make sense when prefixed by "svn: Internal
diff: ". I can't guarantee that for all versions of APR.

Good enough?

- Julian